The invention relates to a Bloch line magnetic memory permitting the storage of binary information.
The principle of the Bloch line memory was proposed in 1983 by S. Konishi in EP-A-0106358 and the article in IEEE Transactions on Magnetics, vol. MAG19, 1983, no. 5, pp. 1838-1840 "A new ultra-high density solid state memory: Bloch line memory".
bloch line memories use the same material as magnetic bubble memories, but have a much greater storage density than bubble memories.
In known manner and as diagrammatically shown in section in FIG. 1, Bloch line memories have a magnetic film 2 of thickness h, located on an amagnetic substrate 4 generally of gallium and gadolinium garnet (GGG). The principal surface 7 of the magnetic film 2 is parallel to the plane xy and its easy magnetization axis F is oriented in accordance with the axis z perpendicular to said principal surface 7.
This magnetic film 2 contains strip or stripe domains 6 and the Bloch lines 8a, 8b of the memory shown in greater detail in FIG. 2. It supports several electrical conductor levels such as 11 and 13 necessary for the operation of the memory. In particular, the upper level 13 incorporates cutting conductors 10 for the strip domains.
These cutting conductors are used both for the reading and writing of an information. They in particular make it possible to discriminate between an easy cut and a difficult cut of the strip domains as a function of the presence or absence of a pair of Bloch lines respectively corresponding to the information "1" or "0". This cut discrimination is consequently one of the essential operations of a Bloch line magnetic memory.
An electrical insulator 12, called a spacer, is located between the electrical conductors of the lower level and the magnetic field 2. In the same way, an electrical insulator 15 or spacer is located between the different levels of conductors. The distance separating the cutting conductors 10 and the film 2 is designated D.
Each strip domain 6 is surrounded by a wall 14 having a certain thickness and their "stabilization" in the magnetic film is ensured by stabilization zones 16 more particularly formed by ionic implantation or etching of the film 2 over its entire thickness. The stabilization of the strip domains is in particular described in the article by H. Kawahara et al, IEEE Transations on Magnetics, vol. MAG. 23, no. 5, September 1987, pp. 3396-3398, "A new method to stabilize multi-stripe domains for a Bloch line memory".
The arrows 18 and 20 indicate the magnetization of the wall 14 and their orientation is dependent on the information contained in the domain 6. The arrows 18 are oriented clockwise and the arrows 20 counterclockwise. The magnetization in the strip domains is opposite to that of the remainder of the magnetic material.
The Bloch lines appear whenever the magnetization in the wall 14 changes direction and are perpendicular to the surface 7 of the film 2. The Bloch lines are always in pairs, namely a line 8a and a line 8b and must have a negative sign for the operation of the memory. For a pair of Bloch lines 8a, 8b, the magnetization in the wall section 18 between the two lines is opposite to the magnetization of the remainder of the wall 20. The sign of a line 8a, 8b does not correspond to the direction of the arrows shown in the drawings, but instead to the rotation direction of the magnetization on the passage of the line from the wall.
Whatever the magnetization orientation in a domain, a Bloch line is defined +, if the rotation of the magnetization on the passage of the line is added to the rotation of the magnetization in the wall of the domain and a Bloch line is defined - when the rotations of these two magnetization are opposed.
The strip domains 6 have two ends respectively 3 and 5 called the first and second strip heads. They are extendible and retractable in a direction passing through said two ends 3 and 4 when a continuous, appropriate control field Hz is applied thereto, in the direction z and oriented from the upper surface 7 of the film to its lower surface 9. Generally, even when the domains contain no information "1", a negative Bloch line at each end of the strip domain is entered and positioned prior to the opposition of the memory.
The organization of a Bloch line memory is similar to that of a bubble memory. This organization is diagrammatically shown in FIG. 3. It comprises a storage zone 22, an access zone 24 and transfer gates or ports 26 between the storage zone and the access zone.
The storage zone is constituted by parallel strip domains 6, whose walls 14 function as minor loops. The stabilization of the lines and the domains is ensured by raster patterns 25 perpendicular to the domains and raster patterns 16, 27 parallel to the domains.
The access zone 24 generally has two insulated conductors 28, 30 arranged in herringbone manner, a generator 32 and a detector 34 of magnetic bubbles 31 in the vicinity of said conductors.
The informations "1" and "0" respectively correspond to the presence or absence of a pair of Bloch lines 8a, 8b in the storage zone. Generally, the presence and absence of a bubble 31 in the access zone 24 correspond respectively to the informations "1" and "0" for reading and the reverse for writing.
The input, the electrical writing signals are converted into magnetic information by the generator 32. The information is stored in the access zone up to the filling thereof. A parallel transfer via the transfer gates 26 passes the information from the access zone to the strip domains. On writing, the new information must replace the old information, so that the writing is accompanied by erasure.
For reading, the information contained in the strip domains 6 is transferred into the access zone for detection. The information contained in the storage zone must not be destroyed. In order to obtain access to a given information, it is necessary that the corresponding bits face the transfer gates 26. The latter must perform two functions, namely the information exchanged during writing and "duplication" during reading.
In known manner, the transfer gates 26 incorporate a checking conductor 36, a strip domain cutting device 10, a writing conductor 38 for the Bloch line pairs which is separate from the device 10 and strip domain extension conductors 40. The cutting device can consist of a conductive hairpin traversed by a current or two parallel conductive strips, which are traversed by currents of the same intensity, but the opposite sense. The reference 10a and 10b represent the two conductive strips (or the two arms of the hairpin) of the cutting device.
The writing conductor 38 can be in the form of a strip, in the manner shown, or can be shaped like a hairpin. Moreover, said conductor 38 can coincide with the cutting device 10. In the latter case, the cutting device 10 is used for both reading and writing.
All the electrical conductors are electrically insulated from one another and their respective shapes and arrangement are variable. In general, the extension conductors constitute the first levels 11 of conductors and the conductors 36, 38 and 10 the other levels 13. The conductors 38, 10, 36 and 40 are respectively connected to electric power supplies 42, 44, 46 and 48.
The writing of a pair of Bloch lines (-, -) corresponding to the information "1" (or transfer in) requires different phases shown in FIGS. 4A to 4B.
In part a of FIG. 4A, the strip domain 6 is at rest and its wall 14 surrounds the stabilization zone 16. An external permanent magnet e.g. creates the polarization field Hz. Patterns 25, oriented perpendicular to the domain 6, make it possible to fix the position of the lines. The Bloch line of the end 3 of the strip domain is designated Lo and is always of sign-.
The first writing phase shown in part b of FIG. 4A consists of extending the strip domain beneath the electrical conductors 36, 38 and 10 by applying an appropriate current i to the extension conductor 40. This current i will be applied throughout the writing period.
Simultaneously with the extension, the checking conductor 36 is activated by a current Ia. The planar magnetic field Hya created below the latter blocks the end line Lo. This current Ia will be applied throughout the writing period. The extended strip domain carries the reference 6a.
This is followed by the actual writing of the pair of Bloch lines (-,-). To do this, writing firstly takes place of an unstable pair of lines (+, -) (part c of FIG. 4A), which is then converted into a stable pair (-,-) (parts a and b of FIG. 4B).
On supplying in known manner a current pulse Ie of approximately 70 mA for 40 ns with a rise and fall time of 10 ns into the writing conductor 38, the locally created magnetic field is adequate for horizontal Bloch loops to be created. If the current Ie is maintained (for 10 to 100 ns, as a function of the magnetic films 2 involved), the loops produced "percent" and the pairs of Bloch lines 50 of opposite polarities (+,-) are nucleated in the wall 14 of the extended domain 6a (part c of FIG. 4A).
Thus, locally the current Ie displaces the wall 14 of the domain. If the local displacement speed of the wall 14 is equal to or greater than the nucleation speed v.sup.o n (said speed only being dependent on the parameters of the magnetic film 2), there is a nucleation of the Bloch loops in the lateral parts 14a or 14b of the wall traversed by the writing conductor 38, on the surface of the film 2 or at the film-conductor interface 2-11. The nucleation of a single pair corresponding to the pair (+,-) 50 is possible if an external planar field Hey is applied. Only the wall whose magnetization is opposite to the direction of the field Hey will have an adequate speed to permit the nucleation of a Bloch loop. In the presence of the planar field Hey, immediately after writing, the lines 50 are separated and the line+ is moved towards the end 3 (FIG. 4B).
In order to convert the pair (+,-) of lines 50 into a stable pair (-,-) 52, a cutting then takes place of the strip domain 6a (part a of FIG. 4B) by applying a current pulse Ic to the in this case hairpin-shaped conductor 10, creating a vertical, local magnetic field Hcz bringing about a local displacement towards one another of the walls 14a and 14b.
By topological continuity of the magnetization vector, a negative line L1 is produced at the end 3 of the domain 6a which has just been cut (part b of FIG. 4B). The magnetic bubble 54 resulting from the cutting of the domain 6a must be evacuated from the access zone and destroyed, because it does not carry any information (part c of FIG, 4B).
Other diagrams can also be considered for writing. For example, a principle proposed by Hitachi in EP-A-0 255 044 is based on the direct writing hypothesis of pairs of negative Bloch lines by the production of a Bloch point.
The invention uses the principle described in FIGS. 4A and 4B.
The return to the rest state of the strip domain 6 (part c of FIG. 4B) is obtained by eliminating the current applied to the extension conductor. In parallel, the current applied to the checking conductor 36 is eliminated.
As has been shown hereinbefore, the nucleation speed V.sup.0 n of the Bloch lines is dependent on the parameters of the magnetic film 2. It can be expressed in the following way: ##EQU1## with a being the gyromagnetic factor of the film, A the magnetic exchange constant, h the magnetic film thickness and Ku the uniaxial anisotropy constant.
This nucleation speed is modified in the presence of a planar magnetic field Hy in the following way: ##EQU2## with .epsilon.=+1 if the magnetization of the film 2 and the field Hy are in the same sense and .epsilon.=-1 if the magnetization and field Hy are of opposite senses.
The principle of reading the informations "1" and "0" stored in the strip domains is shown respectively in FIGS. 5 and 6A-6B. The reading of an information, also known as transfer out, involves several stages.
In part a of FIGS. 5 and 6A, the strip domain 6 is at rest. The reading of an information only requires three types of electrical conductors instead of four in certain cases for writing. These consist of an extension conductor 40, a cutting conductor 10 and a checking conductor 36.
As for the transfer in operation, reading firstly requires the extension of the strip domain 6 of the storage zone 22 towards the transfer zone 26 (part b of FIGS. 5 and 6A) and then the actual reading of the pairs of Bloch lines (parts c of FIGS. 5 and 6A and a and b of FIG. 6B).
The reading of a bit takes place by discrimination between an easy cut (FIG. 5) and a difficult cut (FIGS. 6A-6B) of the strip domain 6a. This reading is a duplication, because following the reading operation, the state of the domain 6 is the same as the initial state. Throughout the reading operation, an appropriate current i is maintained in the extension conductor 40.
At the time of the extension of the strip domain 6a, a current Ia is applied to the checking conductor 36 and is maintained throughout the reading operation, in order to block the Bloch lines of the domain 6a and obtain the two reference wall structures:
wall with parallel magnetization for the easy cut in the presence of a pair of Bloch lines (FIG. 5);
wall with antiparallel magnetization for the difficult cut in the absence of a pair of Bloch lines (FIGS. 6A, 6B).
This is followed by the application of a current pulse Ic (parts c of FIGS. 5 and 6A) to the cutting conductor 10. The presence of Bloch lines permits a low current cutting of the strip domain leading to the formation of a bubble 56, which can be read by the bubble detector 34, whereas the absence of Bloch lines only permits the cutting of the strip domain with higher currents than that necessary for the easy cut.
The application of the current Ic to the cutting conductor 10 creates a local magnetic filed Hcz. When the hairpin-like conductor is positioned perpendicular to the domain 6a to be tested and if the local field Hcz is added to the polarization field Hz of the domain 6a, it is possible to cut the latter. If the magnetizations 18 and 20 of the walls 14a and 14b point in the same direction below the cutting hairpin, it is easy to cut the domain by the latter (FIG. 5). If the magnetizations of the walls 14a and 14b point in opposite directions beneath the cutting hairpin, it is difficult to cut the domain by the latter (FIG. 6A).
The return to the rest state of the strip domain is still obtained by eliminating the current supplied to the extension and checking conductors (parts e and c of FIGS. 5 and 6B).
The cutting margin is defined as the difference between the high current necessary for the difficult cut and lower current necessary for the easy cut.
It is considered that there is no cutting margin if there is an overlap of the current distributions of the two cut types for all the strip domains of the memory. Conversely a cutting margin is obtained when the current distributions of the two cut types for all the domains are separate.
In addition, for a Bloch line memory to function, it is necessary for it to satisfy the "reading" function, i.e. discriminates the easy cuts from the difficult cuts of the domains and consequently has a cutting margin, which is also known as the reading margin.